(321e) Controlling Structure-Property Relationships of Organic Semiconductor Thin Films Using Tunable, Highly-Ordered Self-Assembled Monolayers

Organic semiconductors are competitive with conventional amorphous silicon due to their solution processability at room temperature, which allows for the inexpensive, high-throughput fabrication of lightweight, flexible optoelectronic and electronic devices. Organic semiconductors are applicable in devices such as organic light emitting diodes, organic solar cells, and organic field effect transistors. An inherent disadvantage of organic semiconductors is their propensity to adopt multiple molecular packing motifs, a phenomenon called polymorphism, which can yield highly-contrasting charge transport capabilities and optoelectronic characteristics. More specifically, singlet fission, a photophysical process whereby one exciton is converted into two, has a significant dependence on molecular packing. However, controlling the molecular packing of organic semiconductors is difficult because the underlying crystal structure is dictated by weak Van der Waals forces, and thus, variability in the molecular packing arises from even minute changes in processing conditions.

In this study, the molecular packing of TIPS-pentacene (a small molecule organic semiconductor) was finely-tuned using highly-ordered, tunable self-assembled monolayers. TIPS-pentacene thin films were deposited from solution using solution shearing and subsequently relaxed on top of five ordered self-assembled monolayers. Through this method, various interface-stabilized polymorphs of TIPS-pentacene were isolated. Grazing incidence X-ray diffraction was used to characterize the distinct crystal structures, revealing repeatable and controllable, precise structural changes spanning less than 2% shift from bulk crystal structure. Photoluminescence spectroscopy and UV-Vis spectroscopy were used to discern structure-property relationships. We discovered changes in the bandgap transition energies as well as relative changes to the steady-state photoluminescence, suggesting that these interface-stabilized polymorphs have significant differences in intermolecular interactions.

Singlet fission is an exciting area of study that could result in the breaking of the Shockley-Queisser limit, be the basis for next generation solar cells, and is strongly influenced by intermolecular interactions. Singlet fission rates as a function of polymorph have been studied by other researchers; however, the polymorphs had significantly different crystal structures. Here, our having isolated finely-tuned interface-stabilized polymorphs puts us in a unique position to obtain a deeper understanding of the sensitivity of singlet fission on crystal structure by experimental means through the systematic structural changes. In conclusion, this work provides a novel method to control and explore structure-property relationships through the fine tuning of the molecular packing of small molecule organic semiconductors using highly-ordered, tunable self-assembled monolayers.